Many different types of exercising machines have been developed for use in the field of body building. These machines provide a linear resistance or force against which the user works when repeating a series of single-axis exercises with pivotal movement of a single body member about a single body joint. However, this type of machine provides a uniform amount of resistance or force throughout the exercise movement but does not provide the proper type of variable resistance throughout the entire range of possible movement of the body member.
A significant improvement was achieved in this type of single-axis exercise machine with the invention by the present applicant of the variable resistance exercising machine disclosed in U.S. Pat. No. 3,858,873. The machine of this patent employs spiral pulleys with a different length radius (moment arm) at different angular positions therearound and a weight is attached by a flexible connector to a particular location on the outer surface of the spiral pulley so that the resistance to rotation of the spiral pulley varies, depending upon the length of the moment arm formed by the varying radius of the spiral pulley at various rotational positions. The configuration of the spiral pulley is designed so that the proper amount of muscular resistance is provided throughout the full range of pivotal movement of the body part during the single-axis exercise.
The spiral pulley disclosed in my prior patent will be referred to as a positive profile cam in which the weight is connected to a particular location on the positive profile cam by a flexible connector and the positive profile cam provides moment arms of varying lengths as the positive profile cam is rotated and the flexible connector is wound onto and unwound from the positive profile cam. The force applied by the user is applied to a force applying lever having a moment arm which is longer than any moment arm of the positive profile cam so that the user applied force is multiplied by the force applying lever and the positive profile cam. The rotational axis of the positive profile cam is positioned on a common axis with the involved joint of the body. As is known, the available strength of the muscles of the involved body parts varies at different positions of movement during an exercise and the positive profile cam of my prior machine provides a correspondingly varied resistance to provide a balanced resistance over the full range of the single-axis type of exercise.
However, a compound exercise, that is, an exercise involving two or more body parts and two or more body joints, requires a greater amount of resistance, and a wider range of variation in resistance than it is possible to obtain with the positive profile cam used in my prior machine. Also, the variation in resistance required in a compound exercise machine varies so rapidly and to such a large extent, particularly at certain times during the exercise, that a great amount of angular movement must be imparted to the profile cam by a very small amount of movement of the force applying lever, and it is not possible to accomplish this with the positive profile cam used in my prior machine.
Everything else being equal, a compound exercise, such as a barbell squat, is always a much more productive exercise than any single-axis exercise, such as a barbell curl. The compound exercise is more productive because it involves a far greater mass of muscle than the single-axis exercise. For example, a barbell squat exercise involves the largest and most powerful muscles of the body, the buttocks, the large muscle of the frontal thigh, the lower legs, the muscles of the lower back and others. While the barbell squat is a very productive exercise, it can be dangerous and can cause damage to the knee if not properly performed. Because of the manner in which the calf muscle engages the back of the thigh when the leg is bent to its limit at the lowest end of the squat, the normal pivot point at the knee joint is actually moved rearwardly several inches, to the point where the calf muscle engages the back of the thigh. Since the vertical direction of the weight tends to force the buttocks toward the heels, in a direction transversely of the bones in the upper and lower leg, forces are applied in the knee joint which tend to pull the knee joint apart.
Because of the problems pointed out above, it has not been possible to provide a satisfactory machine for the proper type of variable resistance compound exercising of the muscles associated with the limbs, and to permit the productive type of exercise, such as provided by the squat, while eliminating the potential danger of damaging the knee.